tests: add tests for showing stack when merge is involved This patch adds tests of showing stack when merges are involved. This will help us in changing the behaviour and realising that in later patches.

stack: move data computation on the object We gather multiple data about the stack on the object. Let us move this on the object and keep it cached there.

stack: replace 'getstack' with direct call to 'stack' The dedicated function do not add any value. We can update all the callers.